Estimate is to divide total pixel new/total pixel old then go halfway in between.
(2560x1080)/(1680x1050)/2
Performance should thus drop to about 75% of what it was but that's going to VARY between titles. Also, the new monitor is ULTRAWIDE and there's less processing proportional to pixel count when you go wider so it may even be above 80%.
There are also several HIGHER settings that affect the frame rate a lot but don't add much visually usually, but I don't know the game.
So it's not a massive hit.
I personally don't like Ultrawide monitors for gaming. I looked into it, but there wasn't enough support. Some games look weird, or don't even work so you end up with black bars on the left and right.
So do your research on...